elaborate about the use of alternative instance of parsers

[danse]

see #58

[HuwCampbell]

I think I cover this off pretty well in the arguments section

creates an argument accepting any string. To accept an arbitrary number of arguments, combine the argument builder with either the many or some combinator:

some (argument str (metavar "FILES..."))

No?

[danse]

Well in my case i didn't look under Arguments because i was trying to build an option. I think that a bit of redundancy in documenting this wouldn't hurt. Alternatively the topic could be explained in a small dedicated section when introducing the principles underpinning the lib, i had found it helpful when starting off

[HuwCampbell]

I'll add some extra docs.